The rat as an animal model for fetoplacental development: a reappraisal of the post-implantation period.

Following implantation in rodents, the uterine stromal fibroblasts differentiate into densely packed decidual cells. This process, called decidualization, is well-orchestrated and progresses both antimesometrially and mesometrially, creating two regions with distinctive cellular morphologies. In addition, subsequent placental development is dependent on the invasion of the trophoblast, the process intimately linked to the endometrial tissue remodelling and depending largely on the environment created by the decidua; this phenomenon is crucial for the establishment and maintenance of pregnancy. The key mechanisms underlying the maternal tissue remodelling and trophoblast invasion remain poorly understood. The rat, just like human beings, exhibits a highly invasive type of placental development, the haemochorial placentation. For obvious ethical reasons, the studies of endometrial tissue remodelling throughout pregnancy in humans are greatly limited. Although the rat differs somewhat from humans with regards to the implantation process, it is an appropriate model for studying the mechanisms of decidualization as well as subsequent remodelling of the uterine tissues and fetoplacental development. As decidual remodelling is very closely linked to placentation and the maternal-fetal interactions in the rat show several important similarities to human placentation, the morphological alterations occurring during the post-implantation period in the rat have been addressed in the present review.